Generally, in an illumination light source (such as an electric bulb, a fluorescent lamp, and a mercury lamp) powered by an AC power supply, lightness and darkness are repeated according to the change in luminance of the light source occurring at a double frequency of the AC power supply frequency. When under such illumination, a subject image is captured by an imaging element in which the exposure time for each line begins sequentially or by an imaging element in which the exposure time for each point begins sequentially, periodic horizontal stripes, so-called flickers, are caused in the image signal outputted from the imaging element. The period of the periodic horizontal stripes can be obtained from the drive frequency of the imaging element and the power supply frequency.
For example, since a common fluorescent lamp powered by a 50 Hz AC power supply repeats light emission at 100 Hz, the horizontal scanning (hereinafter referred to as line) frequency in the NTSC system is 15.75 kHz, and hence lightness and darkness are repeated at every 157.5 (= 1/100 sec) lines. Further, since a least common multiple between the field period ( 1/59.94 sec) and the illumination flicker period ( 1/100 sec) is 1/20 sec, the same light and dark pattern is formed every 1/20 sec, that is, about every 3 fields.
Further, since a common fluorescent lamp powered by a 60 Hz AC power supply repeats light emission at 120 Hz, lightness and darkness are repeated at every 131.3 (= 1/120 sec) lines. Further, since the deviation between the field period ( 1/59.94 sec) and the illumination flicker period ( 1/120 sec) is 16.67 μsec, the field period and the flicker period coincide with each other every 500 fields, and hence the flickers appear as horizontal stripes moving very slowly on the screen. The deterioration of image quality due to such illumination flicker becomes an obstacle to viewing.
In order to cope with such problem, a technique is disclosed in Patent Literature 1, in which line average values are stored in a memory for each one third phase of the period between subsequent coincidences between the field period and the flicker period, and in which a reference value, from which the influence of flicker is removed, is created by averaging the average values stored in the memory, and flicker correction is performed on the basis of the reference value. Particularly, in the NTSC system affected by flicker of 60 Hz, a flicker reduction device, which is hardly affected by movement of a subject, is realized in such a manner that, when the subject is stationary, flicker correction is performed, and that, when the subject is moving, the flicker correction is not performed.